To determine the percentage of water in a hydrated salt, a student heated a 1.2346 g sample of the s — Stoichiometry Chemistry Question

STEPS:

1. Identify the Experimental Goal: The objective of heating a hydrated salt is to provide enough thermal energy to break the bonds holding water molecules within the crystal lattice, thereby driving off all "water of hydration" to obtain the anhydrous salt.
2. Analyze the Provided Data: The student began with 1.2346 g of hydrated salt. After 30 minutes of heating, the mass was 1.1857 g. After an additional 10 minutes of heating, the mass dropped further to 1.1632 g.
3. Evaluate the Change in Mass: Subtract the second weighing from the first: $1.1857\text{ g} - 1.1632\text{ g} = \mathbf{0.0225\text{ g}}$. This significant decrease indicates that a substantial amount of water was still being released during the second heating period.
4. Apply the Concept of "Constant Mass": In gravimetric analysis, a sample must be "heated to constant mass" to ensure the reaction is complete. This involves repeating the heating, cooling, and weighing cycle until two consecutive weighings are the same (usually within 0.001 g), proving that no more water remains to be driven off.
5. Determine the Next Logical Step: Because the mass was still decreasing significantly at the 40-minute mark, the student has not yet reached a constant mass. Therefore, the only scientifically sound next step is to continue reheating the sample until the mass stabilizes (Option D).

WHY_OTHERS_WRONG:

• A: Using the 1.1632 g value is premature. The fact that the mass dropped by 0.0225 g in just the last 10 minutes suggests there is likely more water still trapped in the salt.
• B & C: Repeating the experiment with a new sample is inefficient and unnecessary. The current trial is not "wrong"; it is simply incomplete. The student should finish the first trial before attempting a second.
• E: Averaging the two values is mathematically incorrect for this procedure. These weighings represent points in a downward progression toward the final anhydrous mass, not random variations around a true value. Averaging them would result in a value that still contains a significant error from residual water.